Hydrotreating is a process used mainly in oil refining, the purpose of which is to remove impurities such as, for example, the sulfur contained in petroleum cuts resulting from the distillation of the crude oil. A hydrotreating unit comprises a reactor comprising two feed ducts, one intended for introducing the petroleum cut and the other intended for introducing pressurized hydrogen. The reactor contains a catalyst which facilitates the conversion of the sulfur-containing compounds to hydrogen sulfide H2S.
In such a process, it is necessary to sulfide the hydrotreating catalyst which is, usually, sold in an inactive form consisting of metal oxides of group 6 metals combined with group 9 and/or 10 metals, these oxides being supported on a porous solid such as, for example, an alumina. This sulfidation operation takes place at each catalyst change and its objective is to convert the metal oxides to sulfides which constitute the active species in the hydrotreating reaction. To do this, it is known to bring the catalyst into contact with a source of sulfur, such as dimethyldisulfide (DMDS). Under the effect of a high temperature and pressure, the dimethyldisulfide breaks down to give hydrogen sulfide that reacts with the catalyst to form the desired metal sulfides. A detection of hydrogen sulfide formed during the sulfidation of the catalyst is necessary since it makes it possible to have an estimate of the degree of progression of the sulfidation reaction. Moreover, it is desirable to minimize the amount of hydrogen sulfide emitted during the sulfidation reaction. In addition, the measurement of the hydrogen sulfide concentration is currently carried out by the refinery staff, at best every hour, under safety conditions which may be dangerous, in particular due to the toxicity of the hydrogen sulfide (H2S). A device and a technique have therefore been sought that enable a reliable, more frequent measurement under increased safety conditions.
Devices exist for on-line measurement of the concentration of hydrogen sulfide in off-gases originating from units that oxidize the hydrogen sulfide to sulfur, known as Claus units. These are for example described in documents FR 2 778 743 and FR 2 944 456. However, these devices are designed to be permanently mounted on the plant producing the effluent containing the hydrogen sulfide. They cannot be easily dismantled in order to be rapidly used on another Claus unit.
Document CN 203595659U describes a device for measuring the concentration of hydrogen sulfide in a gas stream, the operating principle of which is based on a laser spectroscopy analysis. However, this device requires the use of a gas for inerting the laser equipment, that is to say a gas which is not an oxidizer with the gas to be analyzed and which renders the measurement device anti-explosive. However, the use of an inerting gas makes the device complex.
A measurement device is therefore sought that is easily transportable from one site to another and which does not require the use of an inerting gas.
Document U.S. Pat. No. 8,163,242 describes a device for measuring the concentration of chemical species contained in gases derived from the decomposition of waste present in landfills. However, this document gives no information on the technique used for specifically measuring the hydrogen sulfide concentration. Moreover, this measurement device appears to require an electrochemical technique, which is not suitable for measurements of high concentrations without requiring a dilution gas. The analysis equipment from this document is “a chemical analyser”, which on principle implies an irreversible chemical reaction, therefore a frequent replacement of the chemical sensors.
Document WO 2014/144038 describes a device for the real-time measurement of the concentration of hydrogen sulfide in a petroleum product hydrotreating unit. This device is transportable and is connected temporarily to the outlet duct of the hydrotreating unit. Preferably, the measurement is based on the chemical reaction that occurs between lead acetate and hydrogen sulfide. Lead acetate is deposited on a paper tape, thus giving the tape a white color. During the chemical reaction, black lead sulfide is formed. The degree of darkness of the paper tape is proportional to the amount of hydrogen sulfide that has passed through the measurement system. This measurement system nevertheless has the following drawbacks:                It requires a diffusion chamber intended to dilute, for example with nitrogen, the gas for which it is desired to measure the hydrogen sulfide concentration. The dilution gas may either originate from a local network, or be conveyed with the transportable analysis device. The fact of using nitrogen derived from a refinery may lead to analysis errors due to the pollution resulting from the processes carried out in the refinery.        The replacement of the used paper with new paper impregnated by lead acetate is carried out by an operator. Yet lead acetate belongs to the chemical substances that are carcinogenic and/or mutagenic and/or or toxic for reproduction (so-called CMR substances). Each paper change exposes the operator to contact with lead acetate, which is a health hazard.        This device does not make it possible to maintain a good accuracy of measurement of the H2S concentration over time, over a broad concentration range extending from 0 to 30 000 ppm.        
This document also indicates that the detection of the hydrogen sulfide may be carried out by an electrochemical method.
Document CN 101782514 discloses a device for measuring the concentration of hydrogen sulfide in a natural gas, before and after desulfurization, this device comprising a portion installed in a fixed manner on a plant in which the gas containing H2S circulates, and a detachable portion that can be connected to this fixed portion. The fixed portion comprises:                a measurement chamber wherein the absorption by the gas of a laser radiation is measured;        a pressure gauge and an expansion valve that make it possible to regulate the pressure of the gas to be analyzed to the working pressure of the measurement chamber.The detachable portion comprises:        a device for producing laser radiation; and        a device that processes the optical signal from the measurement chamber.This portion is detachable since it can be connected to the fixed portion with the aid of two optical fibers. Consequently, in this document, only the portion dedicated to the production of the laser radiation and to the processing of the signal is detachable.        
Therefore, it has long been sought in the prior art to have a transportable device for the accurate continuous measurement of the hydrogen sulfide concentration of an off-gas, generally between 100 ppm and 50 000 ppm by volume, preferably between 100 ppm and 30 000 ppm, which can be connected temporarily to a duct for transporting this off-gas and which does not use dilution gas. Preferably, this device should not require the use of dangerous chemical substances such as, amongst others, CMR (carcinogenic, mutagenic, reprotoxic) substances. More preferably, this device should not require a supply of an inerting gas.